Download Interspecific competition in natural plant

Journal of Experimental Botany, Vol. 50, No. 330, pp. 29–37, January 1999
Interspecific competition in natural plant communities:
mechanisms, trade-offs and plant–soil feedbacks
Rien Aerts1
Department of Systems Ecology, Vrije Universiteit, De Boelelaan 1087, NL-1081 HV Amsterdam,
The Netherlands
Received 14 May 1998; Accepted 21 August 1998
Abstract
Introduction
Interspecific competition in natural plant communities
is highly dependent on nutrient availability. At high
levels of nutrient availability, competition is mainly for
light. As light is a unidirectional resource, highnutrient habitats are dominated by fast-growing perennials with a tall stature and a rather uniform vertical
distribution of leaf area. Moreover, these species have
high turnover rates of leaves and roots and a high
morphological plasticity during the differentiation of
leaves. There is less consensus, however, about the
importance and intensity of interspecific competition
in nutrient-poor environments. It is argued that selection in nutrient-poor habitats is not necessarily on a
high competitive ability for nutrients and a high growth
rate, but rather on traits which reduce nutrient losses
(low tissue nutrient concentrations, slow tissue turnover rates, high nutrient resorption efficiency). Due to
evolutionary trade-offs plants can not maximize both
growth rate and nutrient retention. Thus, the low
growth rate of species from nutrient-poor habitats
should be considered as the consequence of nutrient
retention rather than as a feature on which direct
selection takes place. The contrasting traits of species
from nutrient-poor and nutrient-rich habitats mutually
exclude them from each others’ habitats. Moreover,
these traits have severe consequences for litter
decomposability and thereby also for nutrient cycling.
This leads both in nutrient-poor and nutrient-rich habitats to a positive feedback between plant species
dominance and nutrient availability, thereby promoting
ecosystem stability.
Most plant scientists agree that interspecific competition
is an important determinant of the structure and the
dynamics of plant communities. There is, however, much
less agreement about the mechanisms of interspecific
competition. The literature about competition has long
been dominated by the ‘Grime–Tilman’ debate ( Tilman
1985, 1987, 1988; Tilman and Cowan, 1989; Grime, 1979,
1988; Grime and Hodgson, 1987; Thompson, 1987;
Thompson and Grime, 1988). These authors disagree
about the traits of successful competitors and about the
importance of competition in nutrient-poor environments.
Moreover, there is still much discussion about biomass
allocation patterns of successful competitors and about
the relative importance of above- and below-ground
competition for the outcome of competitive interactions.
Finally, some authors claim that, in nutrient-poor environments, traits which lead to high nutrient retention are
far more important for plant performance than traits
which lead to a high competitive ability for nutrient
uptake (Berendse and Aerts, 1987; Aerts, 1990, 1997a;
Berendse, 1994a, b).
A surprising aspect of many papers on competition is
that it is not specified for which resources species are
competing. This certainly contributes to much of the
confusion about the traits of successful competitors
because it will be shown in this paper that there is a
trade-off between traits which lead to a high fitness in
nutrient-poor environments and traits which lead to success in more fertile environments. Moreover, both suites
of traits have important implications for nutrient cycling
processes which may reinforce patterns of species distributions along gradients of soil fertility.
The aim of this paper is to present an overview of the
traits of successful competitors in nutrient-poor and nutri-
Key words: Competition, growth rate, litter decomposition,
nutrient retention, plant strategies.
1 Fax: +31 20 4447123. E-mail: [email protected]
© Oxford University Press 1999
30
Aerts
ent-rich vascular plant communities in temperate regions,
respectively. As there is much less debate on the traits of
successful competitors in fertile environments compared
with nutrient-poor environments, the main emphasis in
this paper will be on competition for nutrients in nutrientpoor environments. To this end, both the growth of
individuals and the demographics of their populations
have been taken into account.
Is competition important?
In fertile environments, with dense canopies, competition
is mainly for light. As light is a uni-directional resource
the traits of successful light competitors can be summarized as ‘traits leading to overtopping of the neighbours’.
These traits include (1) a robust perennial life form with
a strong capacity to ramify vegetatively throughout the
aerial and edaphic environment, (2) the rapid commitment of captured resources to the construction of new
leaves and roots, (3) high morphological plasticity during
the differentiation of leaves and roots, and (4) rapid
turnover of individual leaves and roots (Grime and
Hodgson, 1987). Moreover, it has been shown that the
spatial arrangement of leaf layers, with relatively more
leaf area in the top-layers of the canopy, may also be an
important determinant of the competitive ability for light
interception (Grime, 1979; Spitters and Aerts, 1983;
Mitchley, 1988; Barnes et al., 1988, 1990; Aerts et al.,
1990).
In nutrient-poor environments, competition is mainly
for nutrients. However, there is much discussion about
the importance of interspecific competition in nutrientpoor environments. This discussion has long been dominated by the ‘Grime–Tilman’ debate ( Tilman, 1985, 1987,
1988; Tilman and Cowan, 1989; Grime, 1979, 1988; Grime
and Hodgson, 1987; Thompson, 1987; Thompson and
Grime, 1988). Grime (1979, 1988) claims that competition
is rather unimportant in nutrient-poor environments and
that the intensity of competition increases with increasing
productivity. Under nutrient-poor conditions, traits leading to a high nutrient retention would be far more
important than a high competitive ability for nutrient
uptake. Tilman (1988) claims that the intensity of competition is constant along soil fertility gradients, but that
the relative importance of above- and below-ground
competition changes. The equilibrial resource ratio hypothesis of Tilman ( Tilman, 1985, 1988) claims that, during
succession, plant species replace one another due to
changing selective forces on allocation patterns. Tilman
postulated that the availability of above-ground resources
( light) and below-ground resources (mostly nutrients) are
naturally inversely related. During succession there would
be a gradual increase in the availability of soil nitrogen,
and due to the increasing biomass, light penetration to
the soil surface would decrease. The dominance of a plant
species should depend on the point along the soil–
resource–light gradient at which this species is a superior
competitor. He postulates that, due to the physical separation of above- and below-ground resources, plants face
an unavoidable trade-off between the abilities to compete
for these resources: in order to obtain a higher portion
of one resource plants must allocate more biomass to
structures involved in the acquisition of that resource at
the expense of allocation of biomass to structures involved
in the acquistion of another resource.
The studies in which these theories were tested provide
only inconclusive evidence. Some studies supported
Tilman’s ideas ( Fowler, 1990; Wilson and Shay, 1990;
Wilson and Tilman, 1991, 1993) and others supported
Grime’s ideas ( Reader, 1990; Aerts et al., 1991; Campbell
and Grime, 1992). However, as pointed out by Grace
(1991, 1995), much of the controversy may be explained
by the different ways by which competition is defined
and/or measured. Moreover, it is questionable if there is
indeed a relationship between the intensity of competition
and the importance of competition in structuring plant
communities ( Welden and Slausen, 1986). Thus, at present there is still much confusion about the importance
of competition in nutrient-poor environments. Clearly,
further experimentation is needed here.
In the remainder of this paper two lines of reasoning
will be used. In the first one it will be assumed that
competition is important in nutrient-poor environments
and then the traits of successful competitors will be
discussed. In the second one it will be assumed that
competition is of secondary importance and the emphasis
will be on traits which reduce nutrient losses.
Competition for different nitrogen sources: a
stabilizing mechanism
In most competition models, nutrient competition is
implicitly considered to be competition for inorganic
forms of nutrients (nitrate, ammonium). In recent years,
however, it has become clear that the uptake of organic
nitrogen compounds by both mycorrhizal and nonmycorrhizal plants is an important pathway in the terrestrial nitrogen cycle (Read, 1991; Chapin et al., 1993;
Kielland, 1994; Northup et al., 1995). The ability of
plants to use this ‘short-cut’ of the N cycle may be of
great adaptive significance in nutrient-poor habitats,
because it potentially gives some plants access to a
nitrogen source of which other species are deprived. This
differential use of soil nitrogen sources may be an important mechanism for niche differentation and thus for
ecosystem stability in nitrogen-poor habitats.
In temperate ecosystems the ability to take up more
complex organic N sources is mainly restricted to plants
with ericoid mycorrhizae (EM ) and ectomycorrhizae
( ECM ) and hardly occurs in species with vesicular-
Competition in natural plant communities
arbuscular mycorrhizae (AM ) and in non-mycorrhizal
plants (Aerts and Chapin, 1999). However, amino acid
uptake has been reported for many species, independent
of the presence or form of mycorrhizal infection. For
example, Näsholm et al. (1998) have shown that
Deschampsia flexuosa, growing in a boreal forest, is
capable of utilizing amino nitrogen. The litter of EM
plants usually has higher concentrations of secondary
compounds than litter from AM plants and nonmycorrhizal plants, which may retard N mineralization
and thus decrease the availability of inorganic N in the
soil (Aerts, 1997b). It has been hypothesized that the use
of differential nitrogen sources by the different mycorrhiza
types may create positive feedbacks between plant species
dominance, litter chemistry and mycorrhiza type.
However, until now there has been hardly any field
evidence for this hypothesis.
Heathlands are suitable ecosystems for investigating
the ecological significance of differential uptake of organic
and inorganic nitrogen sources. In nutrient-poor
heathlands ericoid species (Erica tetralix L., Calluna vulgaris (L.) Hull and Empetrum nigrum L.) predominate
(Aerts and Heil, 1993). These ericoid mycorrhizal species
have the ability to use (complex) organic N sources for
their mineral nutrition, thus making them less dependent
on mineralization of organic matter (Read, 1991). In
these heathlands, the vegetation also contains grasses
such as Deschampsia flexuosa (L.) Trin. and Molinia
caerulea (L.) Moench. These species, with AM, have a
limited capacity to utilize organic N sources. This is a
strong disadvantage under nutrient-poor conditions. This
fascinating mechanism of species coexistence as a result
of differential use of soil N sources, may be disrupted
due to increased levels of atmospheric N deposition (Aerts
and Bobbink, 1998). This results in both higher availability of inorganic N and in an increase of the ratio between
inorganic and organic N in the soil. This may affect the
degree of ericoid mycorrhizal infection, thus depriving
the ericoid species of their relative advantage in nutrientpoor soils, and may increase the competitive ability of
the grasses, because they can now utilize a source
of inorganic N. Clearly, the investigation of this type of
species interactions may significantly contribute to our
understanding of the regulation of species distribution
along soil fertility gradients.
Competition in nutrient-poor environments
Nutrient acquisition
By definition, competition in nutrient-poor environments
is for nutrients. Thus, it is logical to assume that plants
in those environments have a high competitive ability for
nutrient uptake. Is this true? The literature on nutrient
uptake has been dominated by studies performed with
31
agricultural species grown at high levels of soil fertility.
These studies showed that the uptake kinetics of plant
roots are an important determinant of nutrient acquisition. However, as Chapin (1980) already pointed out,
great care should be taken when extrapolating these
results to wild plant species from nutrient-poor environments. Nutrient acquisition in natural, nutrient-poor habitats depends on both physiological and morphological
plant features and on the habitat type (Aerts and Chapin,
1999). Morphological traits are especially important for
the acquisition of slowly diffusing nutrients in the soil
such as phosphate.
Uptake kinetics are usually expressed as the rate of
absorption of a particular mineral nutrient per unit root
mass. High uptake kinetics involves the construction of
extra proton pumps and proteins per unit absorptive root
area (Jackson et al., 1990). It has been shown that in
microsites with high nutrient availability, roots of fastgrowing species react rapidly by increasing their uptake
kinetics (Crick and Grime, 1987; Jackson et al., 1990;
Caldwell et al., 1996). This may lead to a competitive
advantage for fast-growing species, because the soil is
depleted of nutrients before slow-growing species have
access to them. This raises the question why slow-growing
species from nutrient-poor natural habitats generally do
not have high uptake kinetics. The answer is simple: in
nutrient-poor habitats, nutrient availability is on average
low and nutrients from outside the depletion zone have
to diffuse to the roots. This implies that the limiting
factor for nutrient uptake in these environments is not
the uptake kinetics, but the diffusion rate of the ions in
the soil solution. This implies that species with high
uptake kinetics face a disadvantage in nutrient-poor environments, because high uptake kinetics does not lead to
higher nutrient uptake, but it does lead to higher carbon
costs for the construction and maintenance of proton
pumps and proteins. However, the situation may be
different for nutrient-rich patches. In a review of the
responses of wild plants to nutrient patches, Robinson
and Van Vuuren (1998) concluded that the uptake rate
per unit of root in some slow-growers can certainly be as
rapid in response to a nutrient-rich patch as in faster
growing species.
The morphological traits related to nutrient acquistion
vary from those operating at the plant level (shoot–root
ratio) to traits operating at the cellular level (root hair
density). Moreover, the capacity of plant roots to proliferate into nutrient-rich patches is of great adaptive significance (Jackson and Caldwell, 1996; Grime et al., 1997).
All these traits are directed towards overcoming the
constraints on nutrient uptake imposed by the low
diffusion rates of nutrients in the soil solution. To put it
simply: the roots move towards the mineral nutrients
instead of the mineral nutrients moving (slowly) to the
roots. Thus, in low-nutrient habitats the morphological
32
Aerts
plant traits are probably more important for increasing
mineral nutrient uptake than the physiological ones
(Jackson and Caldwell, 1996; Aerts and Chapin, 1999).
In conclusion, species from nutrient-poor habitats are
usually not characterized by high nutrient uptake kinetics,
except in situations where nutrient-rich patches occur
( Robinson and Van Vuuren, 1998).
Biomass allocation and competitive ability for nutrient
uptake
It seems logical to assume that species from nutrient-poor
environments allocate more biomass to their root systems
than do species from more fertile sites (cf. Tilman, 1985,
1988). However, this is not a generally observed pattern.
It appears that there are different evolutionary solutions
to this ecological problem: plants can indeed allocate
more biomass to their root systems in order to increase
nutrient uptake or they can show adaptive changes in
their root morphology by having a higher root length per
unit root mass (SRL). Both adaptations have been found
in several studies (Aerts and Chapin, 1999).
Morphological plasticity in biomass allocation may
increase the competitive ability of a plant over a range of
different resource availabilities (Crick and Grime, 1987;
Tilman, 1988; Grime et al., 1997). This raises the question
how nutrient supply affects biomass allocation patterns.
In a competition study with evergreen and deciduous
heathland species (Aerts et al., 1991), both the evergreens
Erica tetralix and Calluna vulgaris and the perennial
deciduous grass Molinia caerulea allocated relatively more
biomass to the roots at low nutrient supply, thus probably
increasing their competitive ability for below-ground
resources. This phenotypic response is common to all
plant species (Aerts and Chapin, 1999). In the monocultures the percentage decrease of biomass allocation to the
roots in Molinia exceeded that in both evergreens thus
pointing to a higher phenotypic plasticity in the partitioning of biomass between shoots and roots. However,
no general conclusions can be drawn from these results,
because Reynolds and D’Antonio (1996) have shown
that there are no strong interspecific differences in the
plasticity of root allocation among species and functional
groups of species.
Contrary to Tilman’s (1988) resource ratio hypothesis,
the allocation patterns of the heathland species studied
by Aerts et al. (1991) entailed no apparent trade-off
between their competitive abilities for above- and belowground resources. Molinia was a superior competitor for
below-ground resources, but not at the expense of its
competitive ability for above-ground resources, despite
its low leaf biomass, which was less than 10% of total
plant biomass compared to 25–30% for both evergreens.
The lower allocation of biomass to the leaves in Molinia
as compared with Erica and Calluna was compensated
by its higher Specific Leaf Area (SLA: leaf area per unit
leaf mass) (R Aerts, unpublished work). On the other
hand, the lower biomass allocation to the roots of Erica
and Calluna as compared with Molinia was compensated
for by their higher Specific Root Length (Boot, 1989).
Thus, the competitive ability for below-ground resources
is not merely a function of biomass allocation patterns,
but also depends on other morphological characteristics,
notably Specific Root Length. Similar patterns were
observed by Berendse and Elberse (1989), Olff et al.
(1990) and Campbell and Grime (1992).
The relative importance of above- and below-ground
competition
The study on the relative importance of above- and
below-ground competition was initiated by the classical
paper by Donald (1958). He used an experimental design
in which the relative effects of above- and below-ground
competition were measured by comparing full competition
situations with situations in which root and/or shoot
competition was prevented by physically separating roots
and/or shoots by using pots and screens, respectively. In
these studies it was found that there is strong interaction
between root and shoot competition. This approach has
been adopted by numerous authors mainly working with
agricultural species. In an extensive review of studies on
the relative importance of above- and below-ground
competition Wilson (1988) found that below-ground
competition usually affected the balance between the
competing species more than above-ground competition.
Moreover, competitive effects appeared to be more severe
at high levels of resource availability. Aerts et al. (1991)
studied the relation between allocation patterns and competitive ability in three species from heathlands in an
experimental garden using the technique developed by
Donald (1958). They also found that the outcome of the
competitive interactions was triggered by root competition, both at low and at high nutrient supply.
Competition or nutrient retention?
Although it is certainly true that interspecific competition
for nutrients is important in explaining species performance in nutrient-poor environments, the situation is more
complicated. Nutrient-poor ecosystems are usually dominated by slow-growing perennial species which predominantly belong to the evergreens (Monk, 1966; Aerts,
1995). The nutrient balance of species in these habitats
is determined by the balance between nutrient acquisition
and nutrient losses (e.g. due to litter production, herbivory
and leaching). Thus, plants from low-nutrient habitats
can have large internal nutrient pools by having a high
competitive ability for nutrient uptake and/or having low
rates of nutrient loss. As already discussed, these species
Competition in natural plant communities
do not show physiological characteristics which lead to
high uptake kinetics, because nutrient acquisition is determined more by the low diffusion rates of mineral nutrients
in the soil solution. Thus, it is to be expected that there
is strong selection on plant traits which lead to low
nutrient loss rates. Model studies (Aerts and Van der
Peijl, 1993; Berendse, 1994a) also show that low nutrient
loss rates of plant species in habitats where plant growth
is nutrient-limited confer clear advantages: low nutrient
loss rates can theoretically lead to a higher equilibrium
biomass ( Fig. 1) and they lead to competitive replacement
of species with higher nutrient loss rates even when these
species have a higher competitive ability for nutrient
uptake (Berendse, 1994a).
It is indeed found that plant species from nutrient-poor
environments are characterized by numerous features which
reduce nutrient losses, such as long tissue lifespan and low
nutrient concentrations in senesced tissues. As high tissue
lifespan leads to retention of nutrients within plants, it may
be expected that species from nutrient-poor habitats would
adopt this strategy. This hypothesis has been confirmed by
numerous studies (Aerts, 1990; Escudero et al., 1992; Reich
et al., 1992; Ryser and Lambers, 1995; Schläpfer and Ryser,
1996; Eckstein and Karlsson, 1997; Eissenstat and Yanai,
1997). The importance of variation in leaf lifespan for a
wide variety of ecological processes, including those related
to mineral nutrition, has been extensively treated by Reich
et al. (1992).
High nutrient retention by the plant is positively correlated with low nutrient concentrations in senesced tissues.
These could arise because the tissues held low concentrations in the first place or because nutrients were resorbed
efficiently during senescence. The former is certainly a
characteristic trait since growth forms are known to differ
consistently in nutrient concentration. For instance, evergreen and deciduous shrubs and trees have, on a whole
plant basis, lower tissue nutrient concentrations than
forbs and grasses (Shaver and Chapin, 1991). This is
largely because the biomass of the woody plants is mainly
Fig. 1. Simulated long-term biomass dynamics of Calluna vulgaris and
Molinia caerulea which have an equal nutrient use efficiency (NUE),
but which differ in their components of NUE: mean residence time of
nitrogen in the plant (MRT ) and the rate of dry matter production per
unit of nitrogen (A). Redrawn from Aerts and Van der Peijl (1993).
33
stem, trunk and roots, consisting of a high proportion of
carbon-rich molecules per unit mass of tissue. Individuals
are able to hold large absolute quantities of nutrients
because they have high biomass compared to forbs and
grasses, though nutrient concentrations are inevitably
low. Leaf nutrient concentrations are lowest in evergreen
shrubs and trees and highest in forbs (Aerts and Chapin,
1999). As carbon assimilation of a leaf is linearly related
to nitrogen content of the leaf (Hirose and Werger, 1987;
Evans, 1989), the N concentration in leaves has implications for the differential productivity of these growth
forms. The relative importance of resorption is less clear.
Two recent analyses of data in the literature have shown
little difference between growth forms and no nutritional
controls on nutrient resorption (Aerts, 1996; Killingbeck,
1996). The conclusion from both papers was that the low
nutrient concentration per unit leaf matter in evergreens
contributed far more to overall nutrient retention than
did resorption during senescence.
Trade-offs and plant-soil feedbacks
Plant species which are successful in nutrient-poor habitats have different sets of adaptive traits (‘strategies’)
than successful competitors in fertile habitats. The strategy of species from infertile habitats comprises traits
which lead to nutrient retention, whereas the strategy of
species from nutrient-rich habitats comprises traits which
lead to rapid growth and quick capture of both aboveand below-ground resources. The fact that this differentiation occurs between species from habitats differing in
soil fertility strongly suggests that there is a trade-off
between their respective traits. If this were not the case,
then the earth would be occupied by a few ‘super-species’
which would dominate all types of habitats. To put it
differently: these hypothesized trade-offs form one of the
fundamental causes of botanical species diversity on earth.
This raises the question if there is any biological logic
behind these trade-offs? In fact, there is. Species from
nutrient-poor habitats are often characterized by tissues
with slow turnover rates, low concentrations of mineral
nutrients and high concentrations of secondary compounds, which serve amongst other things as a defence
against herbivory (Aerts and Chapin, 1999). All these
traits lead to a low growth rate and/or to a low potential
of resource capture (Grime et al., 1997). An example of
this trade-off is provided by data of Reich et al. (1992)
on the relation between leaf lifespan and a wide variety
of ecological parameters. They found a significant negative relation between leaf lifespan and the Relative
Growth Rate (RGR) of plants ( Fig. 2). On the other
hand, traits which do lead to a high growth rate and to
high rates of resource capture, such as rapid turnover of
leaves and high leaf nutrient concentrations, inevitably
lead to high nutrient loss rates and thus low nutrient
34
Aerts
Fig. 2. Relative growth rate (RGR) per week of seedlings in relation
to leaf lifespan (months). R2=0.61. Redrawn from Reich et al. (1992).
retention. Thus, the hypothesized trade-offs have a clear
and logical biological basis.
It is important to notice that the traits associated with
competitive dominance in habitats differing in soil fertility
may also have effects on ecosystem nutrient cycling. In
nutrient-poor environments, species produce relatively
small amounts of litter due to the long lifespans of the
various tissues. This litter generally has low nutrient
concentrations and high concentrations of secondary
compounds such as lignin and phenolics. In a recent
analysis, Aerts (1997b) showed that litter decomposition
rates are negatively related to the lignin/N ratio in the
litter and positively to the N concentration in the litter.
Thus, species from nutrient-poor environments produce
litter which decomposes slowly and from which only low
amounts of nutrients are released. The opposite holds for
species from fertile environments. Due to their high tissue
turnover rates they produce relatively large amounts of
litter. Moreover, this litter contains relatively high concen-
trations of mineral nutrients and low concentrations of
secondary compounds. As a result, this litter decomposes
relatively quickly and releases large amounts of nutrients.
An example of this mechanism is provided by work on
the interaction between species composition and nutrient
cycling in Dutch heathlands (Aerts and Heil, 1993). In
these heathlands, the ericaceous species Erica tetralix and
Calluna vulgaris dominate the vegetation at low nutrient
availability, but they are replaced by the grasses Molinia
caerulea or Deschampsia flexuosa when nutrient availability increases. These grasses produce more litter (except
Deschampsia) which decomposes faster and releases more
nutrients ( Table 1). Thus, these grass species speed up
the rate of nutrient cycling and thereby create favourable
conditions for their own fitness. This pattern was confirmed by a simulation study of Berendse (1994b) who
demonstrated that the plant traits of evergreens ( low
nutrient loss rates and low litter decomposition rates) can
be favourable under nutrient-limited growth conditions.
Low litter decomposability and the resulting low rate of
nutrient release from that litter, as observed in evergreen
species, can theoretically lead to longer dominance of the
evergreen species (Fig. 3). This implies that the plant
characteristics of evergreens not only reduce nutrient
losses, but may also lead to a higher fitness due to longterm effects on soil fertility and thereby on the competitive
balance between evergreen and deciduous species.
Thus, in nutrient-poor ecosystems the combination of
low productivity (and thus low litter production), and
low litter decomposibility may lead to a low rate of
ecosystem N cycling (Chapin, 1993; Van Breemen, 1993).
This may prevent the invasion of highly competitive
species which are dependent on high N availability (Aerts
and Van der Peijl, 1993; Berendse, 1994a, b). On the
other hand, the traits of species from fertile environments
lead to a high rate of ecosystem N cycling and this
excludes slow-growing and nutrient-conserving species
from these habitats.
Fig. 3. Simulated biomass dynamics of an evergreen species (dashed line) with low nutrient loss rates and a competing deciduous species (solid line)
with high nutrient loss rates during succession on bare soil. The decomposition constant of the deciduous species is held constant (k=0.1), whereas
the decomposition constant (k) of the evergreen species is decreased from 0.2 to 0.05. Lowering the decomposition constant leads to longer
dominance of the evergreen species. Redrawn from Berendse (1994b).
Competition in natural plant communities
35
Table 1. Litter production (g m−2 year−1), decomposition constants (k: year−1) and N mineralization of dominant heathland species
in the Netherlands (after Aerts, 1993)
Wet heathland
Litter production
Total
Decomposition constants
Shoots
Roots
N-mineralization
(g N m−2 year−1)
N-mineralization
(mg N g−1 soil N year−1)
Dry heathland
Erica
Molinia
Calluna
Deschampsia
Molinia
800
2060
730
430
2050
0.10
0.03
0.23
0.29
0.17
0.12
4.4
7.8
6.2
16
29
Conclusions
Interspecific competition is an important determinant of
the structure and dynamics of plant communities.
Currently, there is still much debate on the nature and
the intensity of interspecific competition in nutrient-poor
environments. The experimental data which are available
at this moment do not provide any conclusive evidence
for either the ‘Grime’ or the ‘Tilman’ theory. Current
nutrient competition models only consider competition
for inorganic nitrogen. However, recent data show that
many species are capable of taking up nitrogen in organic
form. Due to this ‘short-cut’ of the terrestrial nitrogen
cycle the current models of interspecific competition
should be revised. However, quantitative data about the
importance of this pathway are lacking.
In nutrient-poor environments, root morphology and
root allocation are generally more important for determining the outcome of competition for nutrients than
nutrient uptake kinetics. An additional factor in this
discussion is that the nutrient dynamics of species in
nutrient-poor habitats is determined by the balance
between nutrient acquisition and nutrient loss rates. Thus,
plants from low-nutrient habitats can have large internal
nutrient pools by having a high competitive ability for
nutrient uptake and/or having low nutrient loss rates.
The data available so far strongly suggest that there is
strong selection on traits which lead to low nutrient loss
rates and less on those which lead to a high competitive
ability for nutrient uptake. Thus, in nutrient-poor ecosystems the combination of low productivity (and thus
low litter production), and low litter decomposibility may
lead to a low rate of ecosystem N cycling. This may
prevent the invasion of highly competitive species which
are dependent on high N availability. On the other hand,
the traits of species from fertile environments lead to a
high rate of ecosystem N cycling and this excludes slowgrowing and nutrient-conserving species from these habitats. From these patterns, it can be concluded that the
strategies of species from nutrient-poor and nutrient-rich
habitats promote long-term ecosystem stability. This is
25
0.34
0.24
0.21
0.37
12.6
10.9
36
35
an important evolutionary consequence of these strategies, although it is questionable if long-term ecosystem
stability is prone to natural selection.
Acknowledgements
This paper was one of a series of invited presentations at a
session on Stabilizing processes in mixed plant communities held
as part of the Society for Experimental Biology Annual Meeting
at York in March 1998. Funding for the session was given by
the Scottish Office Agriculture Environment and Fisheries
Department, the Journal of Experimental Botany and the Society
for Experimental Biology.
References
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